Filtration is a process used to remove one or more materials from a fluid. Filters to accomplish this process are of great importance to industry. Many filtration processes are used to remove contaminants from fluids ranging from drinking water to gasoline and other fuels. Yet other filtration processes are used to separate out desired materials. It is desirable for many applications that filters be capable of being regenerated for reuse.
Filters are used, e.g., in reverse osmosis (RO) systems are used on Navy ships to desalinate seawater to meet potable and high purity water needs aboard ship. Reverse osmosis (RO) is a pressure driven membrane process that is widely used for recovering pure water from saline solutions, such as seawater. In RO treatment, a high hydrostatic pressure is applied to the saline solution forcing the water through the semi-permeable membrane and leaving behind a concentrated brine solution. The rate at which product water is produced is proportional to the pressure driving force, that is, to the pressure of the feed water above its osmotic pressure. The osmotic pressure of normal-salinity seawater is around 400 psi and operating pressures generally range from 800 psi to 1000 psi. Resultant fluxes are of the order of 10 gal/ft2/day.
RO systems have generally replaced distillation as the most cost-effective option for shipboard desalination of seawater. An important reason for this is that the cost of RO has decreased significantly with the development of durable, high flux membranes and relatively low-cost modular membrane systems. At the same time, escalating energy prices have increased the cost of thermal distillation which uses five to eight times more fuel than RO (See Pizzino, Joseph F. “Operation of a 2000 gallon per day Reverse Osmosis Desalination System Aboard Monob. David W. Taylor Naval Ship R&D Center, Bethseda, Md. AD-A079 834, December, 1979). To maintain the cost advantage, however, especially for smaller systems with their high costs for pumping and labor (See Kuepper, T. A. “Improved Field Performance for Reverse Osmosis Systems.” Naval Civil Engineering Laboratory, Port Hueneme, Calif. TN No. N-1644. September, 1982), the RO systems must operate at or near their design rating.
The membranes used by the Navy are guaranteed to remove 99.2 percent of the dissolved salts in seawater, and rejections of 99.5 percent or better are typically achieved in practice. Assuming a 99.5 percent rejection and a total dissolved salt concentration of 35,000 mg/L TDS (10,560 mg/L Na) (See Kuepper, T. A. “Improved Field Performance for Reverse Osmosis Systems.” Naval Civil Engineering Laboratory, Port Hueneme, Calif. TN No. N-1644. September, 1982), the permeate (product water) would contain some 175 mg/L of dissolved salts (50 ppm Na), which is within the accepted range for drinking water. However, the Navy also uses treated seawater for their high purity water requirements such as for boiler feedwater, cooling water for electronic equipment, and washdown of gas-turbines (See Adamson, Wayne L.; Weber, Brian E.; Nordham, David J. “Navy Shipboard Three-Pass Reverse Osmosis System for Production of High Purity Water From Seawater.” UltraPure Water 13(2) 21-30, March, 1996). To achieve the extra purity needed for these applications, multistage RO systems have to be used. Such a system is shown in FIG. 1.
In view of the high operating pressures and high pumping costs, any decline in flux is accompanied by a significant increase in production costs. The flux declines progressively under normal operating conditions as the membranes age, requiring that they be replaced periodically, generally every three to five years. However, more rapid decreases in flux can occur due to fouling and scaling of the membrane surface (See Tansel, Barrin; Villate, Jose. “Assessment of Oil Pretreatment Technologies to Improve Performance of Reverse Osmosis Systems.” Florida International University, Miami. AD A252 360, June, 1992). Fouling occurs as the result of the accumulation of particulate matter on the membrane. These particulates form a low-porosity film on the membrane and can also plug the pores of the membrane and/or the narrow flow channels in hollow fiber and spiral wound membrane modules. These problems are aggravated by compaction at the high operating pressures needed for seawater treatment.
To prevent or attenuate fouling and scaling, the seawater is pretreated with prefilters to remove fouling and scaling materials prior to processing in the RO unit. Filtration is the separation of particles from the fluid in which they are suspended by passing the fluid through a septum or filter medium (See Probstein, Ronald F. Physicochemical Hydrodyarnics: An Introduction. 2nd Edition, Prentice Hall, New York, 1996; “Solid-liquid separation via filtration.” Chemical Engineering, 104(2), 66, 1997) The filter can be a screen, a cartridge, a sheet of fabric or canvas, or a bed of granular or fibrous material, and the process may be categorized as straining, cake filtration, membrane filtration, and deep bed or depth filtration. In cake filtration the separated solids are collected on the surface of the filter and in depth filtration the solids are collected within the pores of the medium.
Filtration can occur by four mechanisms: straining, impingement, interception and diffusion (FIG. 2). In mechanical straining 2(a), particles larger than the pores of the medium are retained on the filter. This process is typically used for the removal of relatively large material (generally >150 μm). To remove small particles, other mechanisms are used wherein the pore sizes of filters designed to remove the small particles are generally larger than the particles being removed so as to reduce pressure losses. In impingement 2(b), large; dense particles continue their straight-line trajectory, rather than follow the fluid streamlines, and collide with the filter medium and adhere to it. In interception 2(c), particles that are sufficiently small follow the fluid streamlines, but still come close enough to elements of the filter medium and are collected by the filter. In diffusion 2(d), very small particles (generally <0.3 μm) follow a random trajectory (Brownian motion) that is superimposed on the flow streamlines, and these particles collide with the filter medium.
Liquids flow through the filter by applying a pressure differential across the medium. More porous media are generally more permeable than less porous media, and, thus, require a lower pressure to achieve a given filtration rate. However, the “cutoff” size, i.e., the smallest particle that is just retained by the filter medium, is generally higher for more permeable filter mediums. Considerable filter design efforts have been directed at developing filter media with low cutoff sizes which are still sufficiently permeable to achieve relatively high fluxes without excessive pressure drop.
Cake filtration is one type of filter design wherein the filtered particles collect on the surface of the filter medium. The filter medium may be a single sheet of fabric or a membrane. As filtration progresses, a cake or bed of particles forms on the filter medium. This bed or cake or particles becomes the effective filter medium, and the pressure drop is determined by the characteristics of the particles being filtered. When filtering small particles, the cake will have a low porosity and pressure drops will build up rapidly as the cake thickens. The pressure drop across a cake filter can be described using a modified form of the Hagen-Poiseuille equation (McCabe W. L.; Smith, J. C. Unit Operations of Chemical Engineering. McGraw-Hill. New York. 1976):ΔP=uμ(mR/A+r)  (Eq. 1)where u is the velocity of the filtrate through the filter,m is the mass of the filter cake,A is the cross-sectional area of the filter cakeR is the specific resistance of the filter cake,r is the resistance of the filter medium, andμ is the viscosity of the filtrate.
In the special case of constant rate filtration, the mass of the filter cake increases linearly with time, t, according tom=uAct  (Eq. 2)where c is the mass concentration of the solids in the slurry being filtered.Eq. 1 can then be writtenΔP=u2μctR+uμr  (Eq. 3)
This equation indicates that the pressure drop increases with the slurry concentration as well as with the flow rate. Eq. 3 also shows that provided the cake specific resistance is constant, the pressure drop increases linearly with time. In fact, the cake resistance is a function of the porosity and specific surface area of the cake, and is only constant if the cake is incompressible. With many industrial sludges, the filter cake compresses as the pressure drop increases causing the cake resistance to increase with time, and the pressure drop increases faster than indicated by the equation. The situation is illustrated in FIG. 3, which also indicates how the cake and medium resistance can be estimated from experimental data.
In depth filtration, the particles are trapped in the depths of the filter medium which is relatively thick. In principle, depth filters have pores that decrease in size progressively from the upstream to the downstream face of the filter. Consequently, larger particles are filtered near the upstream face while smaller particles penetrate into the depths of the filter before being trapped. This approach can significantly increase the loading capacity of the filter without excessive increase in pressure drop.
The pretreatment currently used on Navy ships for control of particulates consists of a coarse strainer followed by 20 μm and 3 μm cartridge filters (See Adamson, Wayne L.; Weber, Brian E.; Nordham, David J. “Navy Shipboard Three-Pass Reverse Osmosis System for Production of High Purity Water From Seawater.” UltraPure Water 13(2)21-30, March, 1996). A cyclone separator may be used as an option to provide removal of larger, heavier particulate matter from the incoming seawater.
The cartridge filters used in the prefiltration train for shipboard RO systems are widely used in polishing operations to clarify relatively dilute suspensions or to remove very fine particles from high-purity water streams. These filters typically consist of a central porous core which supports the filter medium, and is contained in a cylindrical housing as shown in FIG. 4. A typical cartridge is 2.5 in. in diameter, 10 in. in height, and is rated for a flow rate of 1 to 20 gpm at a pressure drop, when clean, of around 1 psi. Double length cartridges and housings that hold multiple cartridges are available to meet larger flows. The cartridges are generally considered to be fully loaded when the pressure drop reaches about 10 psi, at which time they are discarded and replaced with fresh cartridges.
Cartridges are available in a wide range of filter media ranging from pleated sheets wrapped around the core to non-woven fiber matrixes which are bonded to the core of the filter. The process of bonding the fiber matrix to the core of the filter involves cooking a “mash” of fibers to produce a pulp that is then drawn under vacuum onto the core to form a coating of filter medium of the desired thickness To provide depth filter properties, the porosity or density of the medium can be graded from the outer surface to the core by appropriately varying the vacuum as the coating thickens.
Although conventional cartridge filters are convenient to use and well suited for polishing type operations, pressure drops increase rapidly and cartridges have to be replaced frequently when handling more concentrated slurries, for example, when operating in regions of the ocean where large quantities of colloidal solids and plankton/small animal matter occasionally exist. In such regions, the strainers and filters plug rapidly, virtually disabling the filtration system. Because these particles have near-neutral buoyancy, cyclonic filters are also ineffective in such ocean regions. The plugged filters must be frequently replaced. This imposes an increased labor burden on operating personnel and results in a loss of production of treated water. In addition, an inventory of replacement filters has to be maintained and the spent filters have to be stored on the ship until the ship returns to port.
Various types of self-cleaning filters that use scrapers, brushes or liquid backwash to regenerate the filter medium are available (See “Solid-liquid separation via filtration.” Chemical Engineering, 104(2), 66, 1997). These self-cleaning filters are useful when the solid content is high and when processing hazardous streams. Cake filters and strainers are generally more amenable to cleaning than are depth type filters.
One filter type that is readily backwashed is the precoat filter, which is a filter having a coating of porous filter aid material such as diatomaceous earth. The precoat layer filters out the particulates from the process stream. To clean the filter, the filtered particles along with some of the precoat can be periodically scraped from the filter surface, or the entire precoat and filter cake can be removed by backwashing. Although precoat filters have been found to be highly effective prefilters for RO systems (See Pizzino, Joseph F. “Evaluation of Single-Pass Seawater Reverse Osmosis Modlules and Pretreatment Techniques—Phase H.” David W. Taylor Naval Ship R&D Center, Bethseda, Md. AD-A035 773, February, 1977; Johns-Mansville Sales Corporation. “Development of a Precoat Filtration Technology for Reverse Osmosis Units.” Final Report AD-A109 053, Nov., 20, 1981), they are generally not suitable for military use because they have excessive labor demands, require a continuous supply of filter aid, and produce copious quantities of waste material that has to be disposed of.
Non-precoat strainer systems with cutoff ratings down to 10 to 15 μm are available (See Anon. “Strainers Find a Niche as Filters.” Chemical Engineering, 102(6), 91-95, 1995) which can be cleaned by scraping as well as by backwashing. They can be designed to operate automatically and thus can reduce labor requirements. Polyvinyl chloride (PVC) units that do not corrode in seawater are on the market. In one design, a circular filter screen is fitted with a circular wiper blade that scrapes collected solids from the screen and pushes them downward where they are removed periodically through a purge at the base of the housing. The cleaning and purging functions are automatic. One unit rated at 60 gpm is 8 in. in diameter, 12 in. long, uses an 80 mesh (177 μm) stainless steel screen, and operates at a pressure drop of 2 to 5 psi (See Shelley, Suzanne. “Self-Cleaning Filter Keeps Pressure Drop Under Control.” Chemical Engineering, 103(7), 149, 1996). Other designs use components which rotate or spin to reduce the rate at which the pressure drop increases and for providing a self-cleaning function. In one such system, a combination of baffles and spinning discs (1000 rpm) keeps the filter surface free of particulates (See Chementator. “Spinning Membranes Resist Fouling During Filtration.” Chemical Engineering, 101(7), 19-20, 1994). Another design consists of three porous wall cylinders located side by side in a single housing (See Chementator. “A Filter that Cleans Itself.” Chemical Engineering, 103(9), 21, 1996). The rapid rotation (2400 rpm) creates a scrubbing action between the cylinders that is claimed to prevent clogging. An alternative approach is to use a vibrating action to repel particulates from the filter medium (See “Filtration Equals Innovation.” Chemical Engineering, 101(10), 119, 1994). Like spinning, vibration increases the shear at the filter surface preventing the accumulation of a dense, low-permeability filter cake.
While the above designs are capable of removing filter build-up, they are not considered suitable for many applications. Further, the moving parts are subject to wear and problems can arise should spinning systems go out of balance. Still further, they require drive motors that occupy space and can fail, and utilize seals on rotating shafts that require regular maintenance. Vibrating systems are unacceptably noisy and can lead to enhanced rate of equipment failure.
What is needed is a filter that may be repeatedly regenerated by simple backwash or forward flush cleaning. In the case of filters used to desalinate seawater aboard a ship, it is desirable to have a filter that is able to meet the potable and high purity water needs aboard the ship and which is capable of filtering a variety of seawater temperatures and salinities.